The present invention relates generally to soft-switching DC/DC converters and, more particularly, to an apparatus for clamping the voltage across a resonant choke to clamp or limit the voltage across the output rectifier devices in a soft-switching DC/DC converter.
Phase shifted, DC/DC bridge converters having zero voltage transition (ZVT) or zero voltage switching (ZVS) are commonly utilized to minimize switching losses. While the typical bridge converter system can be quite satisfactory for many applications, one problem that such bridge converters present is that excess voltage overshoot and severe ringing can occur at the output rectifiers. The output rectifiers are typically embodied as rectifier diodes, but may also be synchronous rectifiers made up of MOSFETs. The excessive overshoot and ringing is caused by the interaction of the reverse recovery process of the rectifier with the leakage inductance of the transformer and can degrade and subsequently cause failure of the output rectifier devices. A snubber circuit does not typically suitably address the excessive overshoot and ringing. Typically clamping of the switches is required.
One proposed solution to this situation can be found with respect to U.S. Pat. No. 5,198,969 to Redl. Redl inserts a pair of clamp diodes in series between the upper and the lower voltage rails. The clamp diodes connect at a node which is between a choke inductor and an output transformer of the bridge converter. In effect, Redl clamps the voltage across the output rectifier devices to the voltage at the junction of the resonant inductor and the power transformer. This effectively clamps the voltage across the primary winding of the transformer and, consequently, the secondary winding of transformer as well. This topology, however, produces a substantial forward current and substantial reverse recovery current in the clamping diodes. This results in substantial power dissipation in the diodes.
An improvement upon the system of Redl can be found with respect to U.S. Ser. No. 09/798,186, filed Mar. 1, 2001, entitled Passive Voltage Clamp for Rectifier Diodes in a Soft-Switching DC/DC Converter, naming Guerrera as an inventor, which is assigned to the assignee of the present invention, the disclosure of which is incorporated by reference. Guerrera clamps both the primary and the secondary voltages at the power transformer by providing an additional winding, referred to as a clamp winding, at the power transformer. While the system of Guerrera proves effective, the manufacture of such a system introduces several challenges. In particular, designing a transformer which can be suitably built to incorporate the Guerrera design proves to be a challenge. Because Guerrera calls for an extra clamp winding in proximity to the primary winding of the transformer, configuring the transformer to include this additional clamp winding can reduce the efficiency of the transformer.
Thus, it is desirable to provide a system which limits excessive voltage overshoot and severe ringing at the output rectifier diodes while maintaining a relatively efficient, manufacturable ZVS/ZVT bridge converter.
This invention is directed to a soft-switching DC/DC converter. The converter includes a positive voltage input terminal and a negative voltage input terminal interconnected by a semiconductor switching device. A transformer of the converter includes a primary and a secondary winding, and an output rectifier circuit is coupled to the secondary winding of the transformer. The converter includes an apparatus for clamping the voltage across the output rectifier circuit. The apparatus includes first and second clamp diodes connected in series across the positive and negative voltage input terminals. A resonant inductor is coupled in series with a primary winding of the transformer, and a voltage clamp circuit is connected across the resonant inductor. The voltage clamp circuit limits the voltage at the output rectifier circuit.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.